1. Field of the Invention
The present invention relates generally to apparatus for cutting deep slots in earth formations, and more particularly the invention relates to apparatus for cutting deep, slightly inclined slots to aid in the strip mining of shallow mineral deposits, particularly coal, without removal of overburden.
2. Description of the Prior Art
A strip mine is an open cut mine in which the overburden is removed from a bed of minerals such as coal before the mineral deposit is taken out. Large scale strip mining operations are carried out over many areas of the world in which an abundance of coal is deposited at a shallow depth below the surface of the earth. These coal deposits represent a potential source of relatively easily accessible and inexpensive fuel. The primary means of recovering these shallow coal deposits is by strip mining, that is, removing the overburden including top soil and foliage and then digging directly into the coal deposits. The removal of the overburden may be accomplished by various methods of excavation, but the basic operation consists of stripping the overburden to expose the mineral deposit and then depositing the stripped overburden in a mound in the area previously mined. The removal of the overburden, in strip mining operations, is generally performed by scraper type earth moving machines or bucket type cranes. Therefore, the major expense in strip mining is the excavation and removal of large volumes of overburden materials. After the minerals have been removed, an attempt is made to restore the natural contour of the land by replacing the overburden materials. However, the inversion of the overburden with respect to the top soil exposes pyrites and other metallic ores to rapid oxidation thereby weakening and leaching the soil and polluting adjoining waterways for many years.
Because of the expense and environmental degradation associated with removal of the overburden, strip mining operations are carried out over disjointed strip pit areas which coincide with concentrated coal deposits, thereby maximizing the ratio of minerals recovered to overburden removed while also minimizing the surface area disturbed. For such operations, narrow coal seams may interconnect the disjoined strip pits, or may lie contiguous with large coal deposits. For the reasons discussed above, it is neither economical nor desirable to remove the overburden from such narrow seams. Additionally, the operating expense for the rather large bucket machines or drag lines makes the cost of recovery for the narrow coal seams prohibitive.
Increasing energy demands require that all known energy sources be fully utilized. Therefore apparatus for carrying out strip mining operations are continuously being improved in order to economically and safely recover the vast coal deposits which lie at a shallow depth over many areas of the world. One such approach is to utilize the methods of "long wall mining", a technique which is limited to very deep coal beds where bed rock makes support relatively easy. Instead of removing the overburden, a narrow trench is dug alongside the coal seam and a long tunnel is dug horizontally along one end of the seam to form a passageway for setting up long wall machinery, including cutters, hydraulic rib supports and conveyors. As the cutters and conveyors work along the wall of the coal seam, new supports are erected next to the cutters and the old supports are removed thereby allowing the roof to cave in and the overburden to drop. Thus a considerable length of coal seam is mined but only a narrow trench need be refilled after the operation has been completed. Mining apparatus which is suitable for practicing the long wall mining technique is disclosed in U.S. Pat. No. 4,014,574. However, the long wall mining technique is still in the experimental stage and it is yet to be seen whether long wall mining can be cost effective for recovering shallow coal deposits.